1. Field of the Invention
This invention relates to marine seismic surveys and more particularly to marine seismic surveys utilizing shear wave boundary reflections.
2. Description of the Prior Art
Conventional marine seismic surveys are developed utilizing one or more marine seismic sources and one or more streamer receiving cables having a plurality of hydrophones or hydrophone arrays spaced therealong at predetermined intervals. The seismic sources are conventionally towed by the same or different marine vessel towing the streamer cables, the sources being towed at a shallow depth beneath the sea's surface to avoid excessive interference from the water-to-air interface or boundary. Likewise, the streamer cables are towed beneath the surface to avoid excessive interference from the surface boundary. The streamer cables are usually towed so that they attain a predetermined attitude, usually parallel to the water's surface.
Each hydrophone or hydrophone array constitutes a suitable detector or receiver of compressional seismic waves propagated through the water as a result of sourcing events. Although there are some refracted compressional waves, these are generally discriminated against in favor of receiving the reflected compressional wave by the alignment of the pressure sensitive elements in the hydrophones. Reflective compressional waves are generally referred to as "p waves", the "p" standing for pressure.
It is well known that geologic boundaries including the sea's bottom and deeper subsurface terrain boundaries, reflect an incident seismic wave from a sourcing event to produce a reflecting p wave or p-wave component. The angle of reflection will generally be equal to the angle of incidence in a formation that is generally homogeneous.
Therefore, in conventional marine surveying, a seismic sourcing event initiates a compressional wave front through the water toward the subsurface terrain to be surveyed. At the sea's bottom there is some refraction of the wave front and some reflection. The refracted wave continues onward to result in sub-surface reflections at each terrain boundary. The bottom reflection is returned at the reflected angle for a p wave to be detected by the hydrophones in the streamer cable. A p-wave reflection from a subsurface boundary progresses upward to the sea's bottom, at which time the ground propagated p wave excites the water in like fashion to produce a water-propagated p wave that is eventually sensed by appropriately positioned hydrophones. From these p-wave returns, the character of the sub-surface terrain surveyed can be determined.
Land surveys, in contrast to marine surveys, utilize geophones instead of hydrophones. Geophones are velocity detectors or receivers, rather than pressure detectors, and are well-suited to detect velocity changes resulting from reflected seismic events. A geophone element is also direction sensitive to discriminate against often unwanted refracted waves. A geophone, however, is unlike a hydrophone in an additional manner, that being it is sensitive to velocity changes as opposed to pressure changes. To be an effective detector, a geophone must either be in contact with the ground surface or just below it and, therefore, be in contact with the propagation medium in which the reflection waves travel.
It is well known that meaningful and different seismic information is derived from the reflected p waves and from so called "shear" or "s" waves. In a land survey, it has long been known that particles are not only excited in an in-line (mostly vertical) direction by a seismic wave front, but also in a transverse (mostly horizontal) direction by a seismic wave front. Shear waves are detectable and can be discriminated against with respect to p waves because of their time of occurrence (slower than a p wave), their angle of reflectivity (less than the angle of reflectivity of a p wave), and by the direction of excited particle motion (which is detectable only by a receiver element oriented for such detection direction).
Shear waves result in land surveys because of the character of layers that naturally occur in the ground. Generally, when seismic energy impinges on an interface between two elastic media having different acoustic impedances, part of that energy will be transmitted across the interface, and part will be reflected back. If the energy arrives at some angle of incidence other than normal to the interface, then both the reflected and the transmitted energy will have a compressional (p-wave) component and a shear (s-wave) component. The partitioning of energy from the incident wave into these components depends upon the angle of incidence and the physical properties of the two media-specifically, their p-wave velocities, s-wave velocities, and bulk densities.
Layers do not occur in water like they do in the ground. However, that does not mean that shear waves have not been totally undetectable in a marine environment. It has meant, however, that sensitivity of detection has been greatly diminished. Because s waves do reflect at a different angle than p waves, it is possible to locate a hydrophone to be sensitive to an s wave following a mode conversion to a p wave as it propagates upward at the sea's bottom. That is, the s wave does excite the water to cause a p wave to exist when it strikes the boundary that is the sea's bottom.
The principle drawback of such detection is the loss of sensitivity in the double-mode conversion process. First, s waves are not as large as corresponding p waves at their initiation since they are a result of excitation transverse to wave propagation. At the water boundary, the direction of excitation again is not in line, but transverse, further weakening an already weak signal. As a result, although it is theoretically possible to obtain s-wave information in a marine environment, conventionally such information is foregone.
Laying a streamer cable with hydrophones on the sea's bottom for s-wave detection is not satisfactory for two reason. First, hydrophones are not sensitive to s-wave detection because they have the wrong kind of element for such detection. Their elements are pressure sensitive, not velocity sensitive. Second, a cable lying on the bottom is not sufficiently stationary or coupled to the bottom to satisfactorily detect s waves. Generally, the sea's bottom is not firm and the wave action moves the cable about so that the s waves are not detectable even if the seismic detection elements were sensitive to detect s waves.
Therefore, it is a feature of the present invention to provide an improved marine seismic detection technique capable of detecting s waves, and doing so in a single-mode conversion reflection procedure.
It is another feature of the present invention to provide an improved seismic detection technique capable of detecting s waves through the employment of suitable detectors enclosed in an improved housing coupled to the sea's bottom.